It is known that a conventional water-impermeable sheet material, for instance, a covering sheet, is made from a hydrophobic polymer film or sheet or a fiber sheet coated or impregnated with a hydrophobic polymer material.
In the conventional air-impermeable, water-proof sheet material, one or two surfaces of the sheet material are made from a natural rubber, synthetic rubber or synthetic film-forming polymeric material, for example, polyvinyl chloride. This type of surface of the conventional air-impermeable, water-proof sheet material is highly hydrophobic and, sometimes, water-repellent. Therefore, when the hydrophobic surface of the sheet material is brought into contact with water, the water exhibits a contact angle of 90 degrees or more to the hydrophobic surface of the sheet material. That is, the water cannot spread on the hydrophobic surface or penetrate into the sheet material. Therefore, the water on the hydrophobic surface of the sheet material forms water drops on the hydrophobic surface.
When the above-mentioned type of air impermeable, water-proof sheet material is used as a covering sheet, for example, for a hood of truck or boat or a tent, in order to protect people or articles from rain or dew it is frequently found that a reduction in the atmospheric temperature causes moisture in a space covered by the covering sheet to be condensed so as to form water drops on the lower surface of the covering sheet. Since the lower surface of the covering sheet is usually highly hydrophobic and, sometimes, water-repellent, the water drops cannot spread on the lower surface and penetrate into the covering sheet. The water drops on the lower surface of the covering sheet are incorporated into each other which increases their size and weight, and, finally, the water drops, having an increased size and weight thereof, are released and fall down gravitationally from the lower surface onto the articles or people.
Accordingly, it is desirable to provide a water-impermeable sheet material which allows the condensed water on a surface thereof to spread on the surface so as to form no water drops, or which allows the condensed water on a surface thereof to penetrate into the surface layer. However, it has been believed that a water-impermeable sheet material must be provided with at least one hydrophobic and, sometimes, water-repellent surface thereof. This is because it has been believed that only the hydrophobic surface of the sheet material can cause the sheet material to be water-impermeable. Therefore, nobody has attempted to provide a water-impermeable sheet material having a non-hydrophobic surface thereof.
Also, it is known that the hydrophobic surface of the conventional water-impermeable sheet material is lipophilic and tends to be easily soiled with oily substances. The oily substances tend to not only adhere to the hydrophobic surface of the sheet material but also, to diffuse into the surface layer of the sheet material. The oily substances are highly hydrophobic and insoluble in water. Therefore, it is very difficult to remove or wash away the oily substances from the hydrophobic surface of the sheet material by utilizing water or rain.
Accordingly, it is desirable to provide a water-impermeable sheet material having a water-impermeable surface which exhibits no affinity with the oily substances and, therefore, highly resistive to being soiled with oily substances.
If the water-impermeable surface of the sheet material is hydrophilic, the hydrophilic surface will be readily received thereon hydrophilic substances and, therefore, will be easily soiled with the hydrophilic substances. However, the hydrophilic substances usually exhibit a high affinity with water. Therefore, it is easy to wash away the hydrophilic substances from the hydrophilic surface of the sheet material with water or rain.